I. Field of the Invention
The present invention relates to a metallized film and a capacitor comprising the same.
II. Description of the Related Art
Capacitors using polymer films have been widely used because of their ability to withstand high voltages as well as good temperature and frequency characteristics. Especially, metallized films having vapor-deposited metal layer as electrodes are favorably utilized because of their desirable property called "self-healing".
As a metal for forming the vapor-deposited layer, aluminum (Al) has been commonly selected since it has good corrosion resistance. However, in capacitors operated with alternate current under high voltage, corona discharge generated inside the capacitor makes the Al be oxidized and non-conductive. Consequently, a phenomenon called ".DELTA.C effect" occurs, which means a decrease in the capacity C of the capacitor. In order to prevent .DELTA.C effect caused by corona discharge, zinc (Zn) is often used for metallization. Zinc, however, has a problem of corrosion.
To solve these problems, countermeasures have been proposed in Japanese Patent Publication (Kokoku) No. 63-15737 and U.S. Pat. No. 4,477,858.
Japanese Patent Publication (Kokoku) No. 63-15737 discloses the prevention of oxidation of a vapor-deposited Zn layer by forming an oil layer on the clean surface of the Zn layer immediately after metallization, by means of vapor deposition inside the vacuum metallization chamber, so that water does not make direct contact with the surface of the vapor-deposited Zn layer, thereby preventing corrosion of the deposited Zn layer. However, even with this method, the vapor-deposited Zn layer of the capacitor tends to be hydroxidized when used in a high moisture environment, which induces an increase of tan.delta. or a deterioration of self-healing capability. This in turn causes capacitor break-down due to a phenomenon called "thermal runaway". Especially, in the case where capacitor film is wound about a soft core and packaged simply with wax or pitch, moisture can get into the capacitor fairly easily, and tends to present the above mentioned problems.
In order to improve corrosion resistance of Zn, U.S. Pat No. 4,477,858 proposes a method where Al is vapor-deposited simultaneously with Zn. By letting the Al content reduce continuously along its thickness from polymer film surface side to the metal layer surface side as shown in FIG. 2, improvements in both moisture resistance and .DELTA.C effect of the capacitors are said to be attained. However, although an improvement in moisture resistance is recognized, .DELTA.C effect due to corona discharge was larger in such capacitors than the ones fabricated form films metallized with Zn alone.